The present invention relates generally to diagnostic imaging and, more particularly, to an apparatus and method of acquiring imaging data at more than one energy range using a multi-energy imaging source.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped or cone-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis, which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom. Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
A CT imaging system may include an energy sensitive (ES), multi-energy (ME), and/or dual-energy (DE) CT imaging system that may be referred to as an ESCT, MECT, and/or DECT imaging system, in order to acquire data for material decomposition or effective Z or monochromatic image estimation. ESCT/MECT/DECT provides energy discrimination. For example, in the absence of object scatter, the system derives the material attenuation at a different energy based on the signal from two relative regions of photon energy from the spectrum: the low-energy and the high-energy portions of the incident x-ray spectrum. In a given energy region relevant to medical CT, two physical processes dominate the x-ray attenuation: (1) Compton scatter and the (2) photoelectric effect. These two processes are sensitive to the photon energy and hence each of the atomic elements has a unique energy sensitive attenuation signature. Therefore, the detected signals from two energy regions provide sufficient information to resolve the energy dependence of the material being imaged. Furthermore, detected signals from the two energy regions provide sufficient information to determine the materials attenuation coefficients in terms of Compton scatter and photoelectric effect. Alternatively, the material attenuation may be expressed as the relative composition of an object composed of two hypothetical materials, or the density and effective atomic number with the scanned object. As understood in the art, using a mathematical change of basis, energy sensitive attenuation can be expressed in terms of two base materials, densities, effective Z number, or as two monochromatic representations having different keV.
Such systems may use a direct conversion detector material in lieu of a scintillator. The ESCT, MECT, and/or DECT imaging system in an example is configured to be responsive to different x-ray spectra. Energy sensitive detectors may be used such that each x-ray photon reaching the detector is recorded with its photon energy. One technique to acquire projection data for material decomposition includes using energy sensitive detectors, such as a CZT or other direct conversion material having electronically pixelated structures or anodes attached thereto. However, such systems typically include additional cost and complexity of operation in order separate and distinguish energy content of each received x-ray photon.
In an alternative, a conventional scintillator-based third-generation CT system may be used to provide energy separation measurements. Such systems may acquire projections sequentially at different peak kilovoltage (kVp) operating levels of the x-ray tube, which changes the peak and spectrum of energy of the incident photons comprising the emitted x-ray beams. A principle objective of scanning with two distinctive energy spectra is to obtain diagnostic CT images that enhance information (contrast separation, material specificity, etc.) within the image by utilizing two scans at different polychromatic energy states.
A number of techniques have been proposed to achieve energy sensitive scanning including acquiring two scans at, for instance, 80 kVp and 140 kVp (1) back-to-back sequentially in time where the scans require two rotations of the gantry around the subject that may be hundreds of milliseconds to seconds apart, (2) interleaved as a function of the rotation angle requiring one rotation around the subject, or (3) using a two tube/two detector system with the tubes/detectors mounted ˜90 degrees apart, as examples. However, taking separate scans several seconds apart from one another may result in mis-registration between datasets caused by patient motion (both external patient motion and internal organ motion) and different cone angles, and cannot be applied reliably where small details need to be resolved for body features that are in motion. A ˜90 degree separation in a two tube/two detector system inherently includes a mis-registration of datasets and adds cost and complexity to the overall system.
High frequency, low capacitance generators have made it possible to switch the kVp potential of the high frequency electromagnetic energy projection source on alternating views and interleave datasets. As a result, data for two energy sensitive scans may be obtained in a temporally interleaved fashion rather than with separate scans made several seconds apart or with a two tube/two detector system. However, such systems typically include a change to filament current to account for a changing mAs when kVp potential is switched. The change in filament current can cause a change in filament temperature which, in turn, can cause a change in focal spot position and/or size. Tube voltage may be used in establishing focal spot width with kVp switching, resulting in an oscillating focal spot width. Such changes can cause low and high projections to be misaligned for material decomposition, causing image artifacts that may be manifested particularly at object edges and boundaries. The problem is exacerbated by a relatively long thermal time constant of the tube filament when compared to the desired rate of fast kVp switching.
The change in focal spot position may be addressed through re-sampling of imaging data to mitigate the alignment issue. Or, if there is a significant change in focal spot size, a sinogram having the smaller focal spot may be blurred for improved registration between high and low kVp sinograms. However, these mitigation strategies tend to degrade resolution of the final image.
Alternatively, an x-ray source may be constructed having a pair of cathodes therein, each configured to emit electrons toward an anode, and each having a respective filament current associated therewith. Such a system may accomplish fast kVp switching by, for instance, gridding the cathodes for the respective low and high kVp shots, with each cathode having a low and high kVp applied thereto relative to the anode. Though such a system may avoid the necessity of rapidly altering kVp or mA in a single cathode, it is at the expense of system complexity—both of hardware and system operation.
Therefore, it would be desirable to design a low cost and low complexity apparatus and method of fast switching between energy levels and acquiring imaging data at more than one energy range.